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Specialized Circuit Drives 150-V Piezoelectric
Motor Using Low-Voltage Op Amp
ALAN STUMMER | UNIVERSITY OF TORONTO [email protected]
A PIEZOELECTRIC MOTOR is a linear motor
with bidirectional motion. It uses friction to grip the armature while a voltage is ramped to warp the piezoelectric
material and move the armature. The
voltage then is quickly removed.
As the material springs back, it breaks
away from the armature and returns to
its zero position, leaving the armature
a few micrometers further along its
track. Repeat this at a kilohertz rate and
for thousands of times. While each of
the motions is very small, after several
seconds you may see that the armature
has moved, if you look carefully. (Full
disclosure: I had never heard of piezo
motors before being asked to make a
driver for one.)
There are two drive waveforms, one
for forward and the other reverse: a sawtooth waveform with a slow linear rise
followed by a fast fall, and its complement with a fast rise and slow linear fall.
This was done using an op-amp triangle-wave oscillator at 1 kHz, with diodes
switched in to speed up both the rising and falling edges to about 5% of the
cycle. The driver’s required bandwidth
is only 10 to 15 kHz.
The problem is the voltage. Fortunately, it is unipolar. Unfortunately, it
is +150 V(peak). The required current
is quite small. It has to be only enough
to charge and discharge the 20-nF piezo
element. A calculation using charge
transfer (Q) shows that during the ramp
phase:
Q = It = CV
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where:
t = 1 ms
C = 20 nF
V = 150 V
therefore:
I = CV/t = 3 mA.
A 600-mW boost converter can be
used to switch the +12 V up to +200
V with a 3-mA load requirement. The
most straightforward circuit uses an op
amp rated to at least 200 V. While some
op amps are available with this voltage
rating, they are meant for high-current
applications and are quite expensive.
The circuit in the figure is much
cheaper and based on a common op
amp used as a non-inverting amplifier.
The heart of the circuit is the current
mirror of R7, N channel FET Q3, and
series-connected R4, R5, and R6. (The
reason for using three resistors in series
is explained below.)
With Q3 in a common-gate configuration and the gate at +12 V, its source
voltage remains reasonably constant at
+10 V (from +12 V less the Vgs(on)). Any
output of op-amp IC1 less than that +10
V causes a voltage drop across R7, with
the current coming from R4 to R6. Since
R7 has the same current as R4 to R6,
there is a voltage gain of the ratio of (R4
to R6)/R7 or 33. The voltage at the bottom of R4 to R6 (the Q3 drain) now has
the required voltage swing but a high
impedance.
A pair of complimentary FETs acts
as followers to lower the output impedance and boost current output. Negative feedback is provided via R2 and R3,
along with bandwidth limiting by C1.
The overall closed-loop gain is set by
(R2 + R3)/R1 + 1 = 16.
There are a few subtleties to the circuit. If something happened to the piezo
motor, such as the user shorting it,
Zener diodes D1 and D2 would protect
the FET gates. The current-mirror highside resistance provided by R4 through
R6 is split into three distinct devices to
handle the power levels so 0805/2012
surface-mount (SMT) resistors can be
used. The feedback resistance composed
of R2 and R3 is split into two devices to
reduce the voltage coefficient of resistance. This is a somewhat obscure effect,
where a component’s resistance actually
changes slightly at higher voltages.
There is a small feedback capacitor
directly on the op amp to provide stability. Without it, the parasitic capacitances
(most notably the Miller capacitances
Cdg and the piezo’s capacitance) would
cause enough phase shift for the op amp
to oscillate. Crossover distortion is not
a problem because the piezo element
could not react to that bandwidth and
does not require a perfect waveform.
Another problem would be the infinite pole caused by a purely capacitive
piezo element. By taking the feedback
directly from the sources of Q1 2, resistor R8 would add a more deterministic
pole to the load and increase loop stabil-
07.11.13 ELECTRONIC DESIGN
ity.
Once the somewhat finicky piezo motor was adjusted, the
final circuit worked well. The armature would move back and
forth with incredibly fine resolution.
ALAN STUMMER designs electronics for the University of Toronto’s
quantum optics and pulse laser research labs, after years working
on analog controls for heavy industry and then telecom.
A low-voltage op amp can be used to drive a higher-voltage piezoelectric motor. D1 and D2 protect the circuit against load shorts, while resistors in series strings are used to reduce individual resistor dissipation
and minimize their voltage coefficient of resistance.
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